Acceleration of grain boundary diffusion during ultra-fast firing (UHS) of alumina powder compacts

The sintering rate of a ceramic at a given temperature and relative density is often higher if the heating to that sintering temperature is more rapid. This has previous been explained in terms of microstructural coarsening during slow heating, which degrades the sinterability of the body before it reaches the sintering temperature. In this work the sintering of pure alumina with conventional heating rates (CS, 100-900 °C/hour) was compared with ultra-fast firing, using the method known as ultrafast high-temperature sintering (UHS, 120-140 °C/s). Despite the wide difference in heating rate, the SEM microstructures for a given density were similar and a modified Master Sintering Curve analysis of the results showed that the small pore size reduction observed was not sufficient to account for the acceleration in sintering of UHS samples compared with CS. There remained an additional acceleration by a factor of up to ∼20 that was attributed to a higher grain boundary diffusion coefficient in UHS. It is suggested that this is the result of the boundaries forming between powder particles not having time to relax to their lowest energy, most compact structures during UHS, i.e. non-equilibrium grain boundaries. In support of this hypothesis, it is shown that powder compacts that were first partially pre-sintered with conventional heating rates and then subjected to UHS did not show the same acceleration of sintering displayed by UHS of green bodies. The implication is that the slow pre-sintering allowed relaxation of the grain boundaries to low energy structures with low diffusion coefficients.